1. Field of the Invention
This invention relates to the field of pulsed magnetic electronic article surveillance (EAS) systems, and in particular, to EAS systems in which the natural frequency of magnetic markers within the EAS system's interrogation zone can be measured for enhanced discrimination against both noise and the presence of magnetically deactivated markers.
2. Description of Related Art
The reliability of an EAS system is only as good as the system's ability to discriminate against false alarms. The potential for false alarms comes from three sources. A first source of false alarms is electromagnetic noise, both internal circuit noise and external noise from active electronic sources. A second source of false alarms is metallic objects which produce magnetic signals when stimulated by the system's transmitter field, but which do not have a valid magnetic marker affixed to them. A third source of false alarms is partially deactivated or damaged magnetic markers or labels within the system's interrogation zone. Such partially deactivated or damaged markers are also referred to as wounded markers.
Conventional amplitude modulation (AM) receivers can only determine the amount of energy within the receiver's bandwidth due to sources within the receiver's field at any point in time. Making an alarm decision based on amplitude criteria alone leaves opportunities for any of the above three sources of false alarms to cause an erroneous system output.
Since EAS systems are invariably expected to operate at maximum sensitivity, the receivers are constantly processing information at poor or marginal signal-to-noise ratios. This is where the chance of making an erroneous decision is greatest. For this reason, the detection process is extended for a longer time interval in these cases, before making a decision. In high noise situations, the extra time required sometimes means a magnetic marker is not within the interrogation zone long enough and the system fails to alarm.
In the case of systems like the Ultra*Max technology systems of Sensormatic Corporation, the response of the magnetic markers or labels is unique enough to consider the second source of false alarms, namely metallic objects which produce magnetic signals when stimulated by the system's transmitter, to be virtually nonexistent. Ultra*Max is a trademark of Sensormatic Corporation.
The Ultra*Max systems are based on a proprietary magneto-acoustic technology. The system's transmitter develops a pulsed magnetic field from its antenna which imparts energy to a unique magnetic marker or label. When the transmission stops, the marker continues to "ring down" for a time based on its unique physical and magnetic properties, in a manner analogous to a struck tuning fork. After the transmission burst, the Ultra*Max receiver scans the interrogation zone to detect the decaying magnetic pulse from an active label.
Deactivating a label or marker involves the demagnetization of a strip of high coercivity bias material within the label. This has the effect of reducing the amplitude of the label's response, as well as shifting the natural resonant frequency of the label upward. Since valid labels are manufactured to have a natural frequency distribution centered at the receiver's operating frequency, typically 58.0 KHz, reducing the label's amplitude and shifting the response frequency out of the receiver's bandwidth has the desired effect of dropping the deactivated label's signal down into the system background noise.
As part of the normal validation sequence, some system designs shift the receiver operating frequency momentarily into a range typical of deactivated labels. At this time the detected amplitude is compared to the amplitude from the previous, standard operating frequency. If the detected amplitude at this deactivated frequency is greater than that detected at the standard operating frequency, the signal is presumed to derive from a deactivated label and no alarm is produced.